Fastener Design Manual, Part One
This manual was written for design engineers to enable them to choose appropriate fasteners for their designs. Subject matter includes fastener material selection, plating, lubricants, corrosion, locking methods, washers, inserts, thread types and classes, fatigue loading, and fastener torque. A section on design criteria covers the derivation of torque formulas, loads on a fastener group, combining simultaneous shear and tension loads, pullout load for tapped holes, grip length, head styles, and fastener strengths. The second half of this manual presents general guidelines and selection criteria for rivets and lockbolts.
To the casual observer the selection of bolts, nuts, and rivets for a design should be a simple task. In reality it is a difficult task, requiring careful consideration of temperature, corrosion, vibration, fatigue, initial preload, and many other factors. The intent of this manual is to present enough data on bolt and rivet materials, finishes, torque, and thread lubricants to enable a designer to make a sensible selection for a particular design. Locknuts, washers, locking methods, inserts, rivets, and tapped holes are also covered.
General Design Information
Bolts can be made from many materials, but most bolts are made of carbon steel, alloy steel, or stainless steel. Stainless steels include both iron- and nickel-based chromium alloys. Titanium and aluminum bolts have limited usage, primarily in the aerospace industry.
Carbon steel is the cheapest and most common bolt material. Most hardware stores sell carbon steel bolts, which are usually zinc plated to resist corrosion. The typical ultimate strength of this bolt material is 55 ksi.
An alloy steel is a high-strength carbon steel that can be heat treated up to 300 ksi. However, it is not corrosion resistant and must therefore have some type of coating to protect it from corrosion. Aerospace alloy steel fasteners are usually cadmium plated for corrosion protection.
Bolts of stainless steel (CRES) are available in a variety of alloys with ultimate strength ranging from 70 to 220 ksi. The major advantage of using CRES is that it normally requires no protective coating and has a wider service temperature range than plain carbon or alloy steels.
A partial listing of bolt materials is given in Table 1. The following precautions are to be noted:
- The bolt plating material is usually the limiting factor on maximum service temperature.
- Carbon steel and alloy steel are unsatisfactory (become brittle) at temperatures below -65 F.
- Hydrogen embrittlement is a problem with most common methods of plating, unless special procedures are used. (This subject is covered more fully in the corrosion section.)
- Series 400 CRES contains only 12 percent chromium and thus will corrode in some environments.
- The contact of dissimilar materials can create galvanic corrosion, which can become a major problem. (Galvanic corrosion is covered later.)
Platings and Coatings
Most plating processes are electrolytic and generate hydrogen. Thus, most plating processes require baking after plating at a temperature well below the decomposition temperature of the plating material to prevent hydrogen embrittlement. However, heating the plating to its decomposition temperature can generate free hydrogen again. Thus, exceeding the safe operating temperature of the plating can cause premature fastener failure due to hydrogen embrittlement as well as loss of corrosion protection. (A summary of plating and coatings is given in Table 2.)
The most common aerospace fastener plating material is cadmium. Plating is done by electrodeposition and is easy to accomplish. However, cadmium-plated parts must be baked at 375 F for 23 hours, within 2 hours after plating, to prevent hydrogen embrittlement. Since cadmium melts at 600 F, its useful service temperature limit is 450 F.
Zinc is also a common type of plating. The hot-dip method of zinc plating is known commercially as galvanizing. Zinc can also be electrodeposited. Because zinc plating has a dull finish, it is less pleasing in appearance than cadmium. However, zinc is a sacrificial material. It will migrate to uncoated areas that have had their plating scratched off, thus continuing to provide corrosion resistance. Zinc may also be applied cold as a zinc-rich paint. Zinc melts at 785 F but has a useful service temperature limit of 250 F. (Its corrosion-inhibiting qualities degrade above 140 F.)
Steel or iron is phosphate coated by treating the material surface with a diluted solution of phosphoric acid, usually by submerging the part in a proprietary bath. The chemical reaction forms a mildly protective layer of crystalline phosphate. The three principal types of phosphate coatings are zinc, iron, and manganese. Phosphate-coated parts can be readily painted, or they can be dipped in oil or wax to improve their corrosion resistance. Fasteners are usually coated with either zinc or manganese phosphate. Hydrogen embrittlement seldom is present in such parts. Phosphate coatings start deteriorating at 225 F (for heavy zinc) to 400 F (for iron phosphate).
Nickel plating, with or without a copper strike (thin plating), is one of the oldest methods of preventing corrosion and improving the appearance of steel and brass. Nickel plating will tarnish unless followed by chromium plating. Nickel plating is more expensive than cadmium or zinc plating, and also must be baked as cadmium to prevent hydrogen embrittlement. Nickel plating is good to an operating temperature of 1100 F, but is still not frequently used for plating fasteners because of its cost.
Ion-Vapor-Deposited Aluminum Plating
Ion-vapor-deposited aluminum plating was developed by McDonnell-Douglas for coating aircraft parts. It has some advantages over cadmium plating:
- It creates no hydrogen embrittlement.
- It insulates against galvanic corrosion of dissimilar materials.
- The coating is acceptable up to 925 F.
- It can also be used for coating titanium and aluminum.
- The process forms no toxic byproducts.
It also has some disadvantages:
- Because the process must be done in a specially designed vacuum chamber, it is quite expensive.
- Cadmium will outperform ion-vapor-deposited aluminum in a salt-spray test.
Chromium plating is commonly used for automotive and appliance decorative applications, but it is not common for fasteners. Chromium-plated fasteners cost approximately as much as stainless steel fasteners. Good chromium plating requires both copper and nickel plating prior to chromium plating. Chromium plating also has hydrogen embrittlement problems. However, it is acceptable for maximum operating temperatures of 800 to 1200 F.
Sermatel W and SermaGard
Sermatel W and SermaGard are proprietary coatings consisting of aluminum particles in an inorganic binder with chromates added to inhibit corrosion. The coating material is covered by AMS3126A, and the application procedure by AMS2506. The coating is sprayed or dipped on the part and cured at 650 F. (SPS Technologies 2 has tested Sermatel W-coated fasteners at 900 F without degradation.) This coating process prevents both hydrogen embrittlement and stress corrosion, since the fastener is completely coated. Sermatel is about as effective as cadmium plating in resisting corrosion but costs about 15 percent more than cadmium. Fasteners are not presently available "off the shelf" with Sermatel W or SermaGard coating, but the company will do small orders for fasteners or mechanical parts. These coatings will take up to 15 disassemblies in a threaded area without serious coating degradation.
Stalgard is a proprietary coating 3 process consisting of organic coatings, inorganic-organic coatings, or both for corrosion resistance. According to Stalgard test data their coatings are superior to either cadmium or zinc plating in salt spray and weathering tests. Stalgard coatings also provide galvanic corrosion protection. However, the maximum operating temperature of these organic coatings is 475 F.
Diffused Nickel-Cadmium Plating
This process was developed by the aerospace industry to allow for higher temperature cadmium coating. A 0.0004-in.-thick nickel coating is plated on the substrate, followed by a 0.0002-in. thick cadmium plate (per AMS2416). The part is then baked for 1 hour at 645 F. The resulting coating can withstand 1000 F. However, the nickel plate must completely cover the part at all times to avoid cadmium damage to the part. This process is expensive and requires close control.
Silver plating is cost prohibitive for most fastener applications. The big exception is in the aerospace industry, where silver-plated nuts are used on stainless steel bolts. The silver serves both as corrosion deterrent and dry lubricant. Silver plating can be used to 1600 F, and thus it is a good high-temperature lubricant. Since silver tarnishes from normal atmospheric exposure, the silver-plated nuts are commonly coated with clear wax to prevent tarnishing. Wax is a good room-temperature lubricant. Therefore, the normal "dry torque" values of the torque tables should be reduced by 50 percent to allow for this lubricant.
Passivation and Preoxidation
Stainless steel fasteners will create galvanic corrosion or oxidation in a joint unless they are passivated or preoxidized prior to assembly (ref. 1). Passivation is the formation of a protective oxide coating on the steel by treating it briefly with an acid. The oxide coating is almost inert. Preoxidization is the formation of an oxide coating by exposing the fasteners to approximately 1300 F temperature in an air furnace. The surface formed is inert enough to prevent galling due to galvanic corrosion.
Black Oxide Coating
Black oxide coating, combined with an oil film, does little more than enhance the appearance of carbon steel fasteners. The oil film is the only part of the coating that prevents corrosion.
There are many thread lubricants from which to choose, but only a few of the most commonly used are covered here. The most common are oil, grease or wax, graphite, and molybdenum disulfide. There are also several proprietary lubricants such as Never-Seez and Synergistic Coatings. Some thread-locking compounds such as Loctite can also be used as lubricants for a bolted assembly, particularly the compounds that allow the bolts to be removed. A summary of thread lubricants is given in Table 3.
Oil and Grease
Although oil and grease are the most common types of thread lubricants, they are limited to an operating temperature not much greater than 250 F. (Above this temperature the oil or grease will melt or boil off.) In addition, oil cannot be used in a vacuum environment. However, oil and grease are good for both lubrication and corrosion prevention as long as these precautions are observed.
"Dry" graphite is really not dry. It is fine carbon powder that needs moisture (usually oil or water) to become a lubricant. Therefore, its maximum operating temperature is limited to the boiling point of the oil or water. It also cannot be used in a vacuum environment without losing its moisture. Because dry graphite is an abrasive, its use is detrimental to the bolted joint if the preceding limitations are exceeded.
Molybdenum disulfide is one of the most popular dry lubricants. It can be used in a vacuum environment but turns to molybdenum trisulfide at approximately 750 F. Molybdenum trisulfide is an abrasive rather than a lubricant.
These proprietary coatings are a type of fluorocarbon injected and baked into a porous metal-matrix coating to provide corrosion prevention and lubrication. However, the maximum operating temperature given in their sales literature is 500 F. Synergistic Coatings will also operate in a vacuum environment.
This proprietary compound is a petroleum-base lubricant and anticorrodant that is satisfactory as a one-time lubricant up to 2200 F, according to the manufacturer. The oil boils off, but the compound leaves nongalling oxides of nickel, copper, and zinc between the threads. This allows the fastener to be removed, but a new application is required each time the fastener is installed. NASA Lewis personnel tested this compound and found it to be satisfactory.
Silver Goop is a proprietary compound containing 20 to 30 percent silver. Silver Goop can be used to 1500 F on materials other than aluminum or magnesium. It is extremely expensive because of its silver content.
Some of the removable thread-locking compounds (such as Loctite) also serve as antigalling and lubricating substances. However, they are epoxies, which have a maximum operating temperature of approximately 275 F.
Galvanic corrosion is set up when two dissimilar metals are in the presence of an electrolyte, such as moisture. A galvanic cell is created and the most active (anode) of the two materials is eroded and deposited on the least active (cathode). Note that the farther apart two materials are in the following list, the greater the galvanic action between them.
According to reference 2 the galvanic ranking of some common engineering materials is as follows:
|Galvanic Ranking of Engineering Materials|
Magnesium (most active)
Type 410 stainless (active)
Type 304 stainless (active)
Type 316 stainless (active)
Type 304 stainless (passive)
Type 316 stainless (passive)
Gold (least active)
Note the difference between active and passive 304 and 316 stainless steels. The difference here is that passivation of stainless steels is done either by oxidizing in an air furnace or treating the surface with an acid to cause an oxide to form. This oxide surface is quite inert in both cases and deters galvanic activity.
Because the anode is eroded in a galvanic cell, it should be the larger mass in the cell. Therefore, it is poor design practice to use carbon steel fasteners in a stainless steel or copper assembly. Stainless steel fasteners can be used in carbon steel assemblies, since the carbon steel mass is the anode.
Magnesium is frequently used in lightweight designs because of its high strength to weight ratio. However, it must be totally insulated from fasteners by an inert coating such as zinc chromate primer to prevent extreme galvanic corrosion. Cadmium- or zinc-plated fasteners are closest to magnesium in the galvanic series and would be the most compatible if the insulation coating were damaged.
Stress corrosion occurs when a tensile-stressed part is placed in a corrosive environment. An otherwise ductile part will fail at a stress much lower than its yield strength because of surface imperfections (usually pits or cracks) created by the corrosive environment. In general, the higher the heat-treating temperature of the material (and the lower the ductility), the more susceptible it is to stress corrosion cracking.
Material manufacturers have been forced to develop alloys that are less sensitive to stress corrosion. Of the stainless steels, A286 is the best fastener material for aerospace usage. It is not susceptible to stress corrosion but usually is produced only up to 160-ksi strength (220-ksi A286 fasteners are available on special order). The higher strength stainless steel fasteners (180 to 220 ksi) are usually made of 17-7PH or 17-4PH, which are stress corrosion susceptible. Fasteners made of superalloys such as Inconel 718 or MP35N are available if cost and schedule are not restricted.
An alternative is to use a high-strength carbon steel (such as H-11 tool steel with an ultimate tensile strength of 300 ksi) and provide corrosion protection. However, it is preferable to use more fasteners of the ordinary variety and strength, if possible, than to use a few high-strength fasteners. High-strength fasteners (greater than 180 ksi) bring on problems such as brittleness, critical flaws, forged heads, cold rolling of threads, and the necessity for stringent quality control procedures. Quality control procedures such as x-ray, dye penetrant, magnetic particle, thread radius, and head radius inspections are commonly used for high-strength fasteners.
Hydrogen embrittlement occurs whenever there is free hydrogen in close association with the metal. Since most plating processes are the electrolytic bath type, free hydrogen is present. There are three types of hydrogen-metal problems: * Hydrogen chemical reaction: Hydrogen reacts with the carbon in steel to form methane gas, which can lead to crack development and strength reduction. Hydrogen can also react with alloying elements such as titanium, niobium, or tantalum to form hydrides. Because the hydrides are not as strong as the parent alloy, they reduce the overall strength of the part. * Internal hydrogen embrittlement: Hydrogen can remain in solution interstitially (between lattices in the grain structure) and can cause delayed failures after proof testing. There is no external indication that the hydrogen is present. * Hydrogen environment embrittlement: This problem is only present in a high-pressure hydrogen environment such as a hydrogen storage tank. Unless a fastener was under stress inside such a pressure vessel, this condition would not be present.
Most plating specifications now state that a plated carbon steel fastener "shall be baked for not less than 23 hours at 375 + 25 F within 2 hours after plating to provide hydrogen embrittlement relief" (per MIL-N-25027D). In the past the plating specifications required baking at 375 + 25 F for only 3 hours within 4 hours after plating. This treatment was found to be inadequate, and most plating specifications were revised in 1981-82 to reflect the longer baking time. Hydrogen embrittlement problems also increase as the fastener strength increases.
Although hydrogen embrittlement failure of materials is well documented (ref. 3), the effects of cadmium embrittlement are not. In general, hydrogen embrittlement failure of cadmium-plated parts can start as low as 325 F, but cadmium embrittlement can start around 400 F. Since both elements are normally present in elevated-temperature failure of cadmium-plated parts, the combined effect of the two can be disastrous. However, the individual effect of each is indeterminate.
Figure 1. Spiralock Thread.
In a tapped hole the locking technique is normally on the fastener. One notable exception is the Spiralock 7 tap shown in Figure 1. The Spiralock thread form has a 30' wedge ram at its root. Under clamp load the crests of the male thread are wedged tightly against the ramp. This makes lateral movement, which causes loosening under vibration, nearly impossible. Independent tests by some of the aerospace companies have indicated that this type of thread is satisfactory for moderate resistance to vibration. The bolt can have standard thread, since the tapped hole does all the locking.
[Spiralock is distributed by Detroit Tap & Tool Company, Detroit, Michigan, through license from H.D. Holmes.]
Figure 2. Split-beam locknut.
There are various types of locking elements, with the common principle being to bind (or wedge) the nut thread to the bolt threads. Some of the more common locknuts are covered here.
Split beam. The split-beam locknut (Figure 2) has slots in the top, and the thread diameter is undersized in the slotted portion. The nut spins freely until the bolt threads get to the slotted area. The split "beam" segments are deflected outward by the bolt, and a friction load results from binding of the mating threads.
Deformed thread. The deformed-thread locknut (Figure 3) is a common locknut, particularly in the aerospace industry. Its advantages are as follows:
Figure 3. Deformed-thread locknut.
- The nut can be formed in one operation.
- The temperature range is limited only by the parent metal, its plating, or both.
- The nut can be reused approximately 10 times before it has to be discarded for loss of locking capability.
Figure 4. Nylok pellet locknut.
Nylok pellet. The Nylok pellet (of nylon) is usually installed in the nut threads as shown in Figure 4. A pellet or patch projects from the threads. When mating threads engage, compression creates a counter-force that results in locking contact. The main drawback of this pellet is that its maximum operating temperature is approximately 250 F. The nylon pellet will also be damaged quickly by reassembly.
Figure 5. Locking collar.
Locking collar and seal. A fiber or nylon washer is mounted in the top of the nut as shown in Figure 5. The collar has an interference fit such that it binds on the bolt threads. It also provides some sealing action from gas and moisture leakage. Once again the limiting feature of this nut is the approximate 250 F temperature limit of the locking collar.
A cost-saving method sometimes used instead of a collar or nylon pellet is to bond a nylon patch on the threads of either the nut or the bolt to get some locking action. This method is also used on short thread lengths, where a drilled hole for a locking pellet could cause severe stress concentration.
Figure 6. Castellated nut.
Castellated nut. The castellated nut normally has six slots as shown in Figure 6 (a). The bolt has a single hole through its threaded end. The nut is torqued to its desired torque value. It is then rotated forward or backward (depending on the user's preference) to the nearest slot that aligns with the drilled hole in the bolt. A cotter pin is then installed to lock the nut in place as shown in Figure 6 (b). This nut works extremely well for low-torque applications such as holding a wheel bearing in place.
Figure 7. Jam nut.
Jam nuts. These nuts are normally "jammed" together as shown in Figure 7, although the "experts" cannot agree on which nut should be on the bottom. However, this type of assembly is too unpredictable to be reliable. If the inner nut is torqued more tightly than the outer nut, the inner nut will yield before the outer nut can pick up its full load. On the other hand, if the outer nut is tightened more than the inner nut, the inner nut unloads. Then the outer nut will yield before the inner nut can pick up its full load. It would be rare to get the correct amount of torque on each nut. A locknut is a much more practical choice than a regular nut and a jam nut. However, a jam nut can be used on a turnbuckle, where it does not carry any of the tension load.
Figure 8. Durlock nut.
Serrated-face nut (or bolthead). The serrated face of this nut (shown in Figure 8) digs into the bearing surface during final tightening. This means that it cannot be used with a washer or on surfaces where scratches or corrosion could be a problem.
According to SPS Technologies, their serrated-face bolts (Durlock 180) require I 10 percent of tightening torque to loosen them. Their tests on these bolts have shown them to have excellent vibration resistance.
Figure 9. Lockwiring.
Lockwiring. Although lockwiring is a laborious method of preventing bolt or nut rotation, it is still used in critical applications, particularly in the aerospace field. The nuts usually have drilled corners, and the bolts either have through-holes in the head or drilled corners to thread the lockwire through. A typical bolthead lockwiring assembly is shown in Figure 9 (a), and a typical nut lockwiring assembly is shown in Figure 9 (b). Direct interfering thread. A direct interfering thread has an oversized root diameter that gives a slight interference fit between the mating threads. It is commonly used on threaded studs for semi-permanent installations, rather than on bolts and nuts, since the interference fit does damage the threads.
Figure 10. Tapered thread.
Tapered thread. The tapered thread is a variation of the direct interfering thread, but the difference is that the minor diameter is tapered to interfere on the last three or four threads of a nut or bolt as shown in Figure 10,
Nutplates. A nutplate (Figure 11) is normally used as a blind nut. They can be fixed or floating. In addition, they can have most of the locking and sealing features of a regular nut. Nutplates are usually used on materials too thin to tap. Primarily the aerospace companies use them, since their installation is expensive. At least three drilled holes and two rivets are required for each nutplate installation.
Figure 11. Nutplate.
Many manufacturers make locking adhesives (or epoxies) for locking threads. Most major manufacturers make several grades of locking adhesive, so that the frequency of disassembly can be matched to the locking capability of the adhesive. For example, Loctite 242 is for removable fasteners, and Loctite 2719 is for tamper-proof fasteners. Other manufacturers such as Bostik, ND Industries, Nylock, 3M, and Permaloc make similar products.
Most of these adhesives work in one of two ways. They are either a single mixture that hardens when it becomes a thin layer in the absence of air or an epoxy in two layers that does not harden until it is mixed and compressed between the mating threads. Note that the two-layer adhesives are usually put on the fastener as a "ribbon" or ring by the manufacturer. These ribbons or rings do have some shelf life, as long as they are not inadvertently mixed or damaged.
These adhesives are usually effective as thread sealers as well. However, none of them will take high temperatures. The best adhesives will function at 450 F; the worst only 200 F.
Move on to part two.